External cavity lasers can exhibit an important advantage of wavelength tuning over large wavelength ranges. An optical gain medium emits light that propagates within the external laser cavity. Wavelength tuning in an external laser cavity depends on the dispersion of light resonating within the cavity. Diffractive focusing elements are incorporated in some external cavity laser designs. In these cases, the dispersion of light either transmitted through or reflected from the diffractive focusing element enables a significant range of wavelength tuning.
Diffractive focusing elements in an external cavity laser are placed either a focal length or two focal lengths from the optical gain medium, e.g., a laser diode, in the case of transmissive and reflective diffractive focusing elements, respectively. Diffractive focusing elements with smaller f number (defined as the focal length divided by diameter) cause larger dispersion, with the largest dispersion occurring at the periphery of the diffractive element. Ideally, light propagating within the cavity exactly fills the diffractive focusing element aperture. However, typical laser diodes emit light with small angular beam divergence. Thus, light incident on a diffractive element of desired small f number, e.g., focal length equal to diameter, may under-fill the aperture of the diffractive element. Under-sampling the highly dispersive diffractive periphery limits the dispersion of light resonating in the cavity. This impairs the laser cavity wavelength tuning performance.